1. Field of the Invention
The present invention relates to a method of molding a disc substrate having an improved structure, and particularly to a method of molding a disc substrate which is improved in quality, and more particularly to a method of molding a disc substrate which allows an optical disc to be manufactured stably and with high moldability.
2. Description of the Related Art
Conventionally, a disc die for manufacturing an optical disc such as a video disc or a compact disc, or the like is so constituted that a predetermined stamper 4 is arranged within a product cavity 3 which is comprised of a movable die 1 and a fixed die 2 as shown in FIG. 4, and the inner and outer peripheries of the stamper 4 are fixed by stamper pressing rings 5 and 6, respectively. Also, a surface 4a of the stamper 4 is formed with spiral or concentric circular irregular pits for transferring record information data onto the disc.
In general, there has been adopted an injection molding method in which molten resin is filled into the cavity from an injection unit after a mold clamping process, and the information of the stamper surface 4a which is exposed from one surface of the cavity 3 is transferred onto a resin material which is to be solidified to thereby mold the disc substrate.
The conventional injection molding method is implemented in such a manner where, in an operating process, for example, as shown in FIG. 5, after the completion of mold closing operation, an internal cavity pressure is intensified, and a series of molding cycle is conducted with each of an injection filling process, an injection pressure holding process, and a cooling process under a state where a mold clamping pressure is held. Also, in FIG. 6, an effort was made to manufacture an improved product in such a manner that the mold clamping pressure is adjusted by controlling a first clamping pressure, a second clamping pressure, or also a multi-stage clamping pressure (3 to n times) upwardly or downwardly, with the setting of a plurality of timers.
In such an injection molding operation for the disc substrate, the clamping force is required to be such a degree that the paired molds are prevented from opening even though a resin material is filled into the mold. For example, the clamping force of 40 tons or more in the case of a disc substrate having a diameter of 120 mm, and the clamping force of 200 tons or more in the case of a disc substrate having a diameter of 300 mm are required, respectively.
The stamper 4 is fixed onto the inside of the cavity 3 in such a manner that the inner peripheral portion of the stamper 4 which is usually in the form of a doughnut is fixed onto the movable die 1 through appropriate fixing means, and the outer peripheral portion thereof is clamped by the stamper pressing ring 6 shown in FIG. 4. A gas removal portion 8 is formed at a given interval with a stamper pressing angle .alpha. of approximately 7.degree., for exhausting gas extruded from the inside of the cavity to the exterior in the filling of a predetermined resin material into the cavity between a mirror plate 7 of the fixed die 2 and the stamper surface 4a.
In the conventional molding method of such a structure, because of a narrowed molding condition, the quality of the resultant products is largely dispersed, and its yield is lowered. In addition, a large injection pressure for filling a predetermined resin material into the product cavity makes a pressure stripe formed on the peripheral portion of the molded disc substrate, as a result of which the quality of the product is dispersed between the inner peripheral portion and the outer peripheral portion of the stamper 4, in particular, there arises a problem that the index of double refraction of the product is changed.
In the molding of the disc substrate, the improvements in a transfer, the index of double refraction, and the mechanical characteristics of a warp, a plane and the like are demanded as important items. However, the molding of a substance which is thin in its thickness and large in its projected area as in the disc substrate makes it difficult to satisfy all of those characteristics. Moreover, the applicability of the disc substrate is expanded in the future, and specification values are made strict such that the thickness of 1. mm at the present time will tend to be still thinner in the future. Also, a study has been made of using two layered disc substrates each having a thickness of 0.6 mm, which have been stuck on one another, etc. Under such existing circumstances, there is a limit to manufacture a product coping with the specification which has been demanded in the conventional molding method.
For that reason, there has been proposed, in Japanese Patent Examined Publication No. Hei 2-19774, an injection molding method for a disc substrate which is characterized as follows: A resin material is injected within a product cavity under a primary injection pressure where a mold opening force which is slightly larger than a mold clamping force is exerted on the movable die so that the quantity of resin necessary for forming a desired disc substrate is injected into the product cavity. At that instant, the injection pressure is changed over to a secondary injection pressure where the mold opening force which is exerted on the movable die is smaller than the mold clamping force, thereby filling the injected resin material into the product cavity through the mold clamping force. Simultaneously, a gas extruded from the product cavity with the filling operation is exhausted from a gap defined between an inner side surface of the stamper pressing member and a rising wall surface of a stepped portion of the fixed die side.
However, in the conventional molding method, after the completion of the mold closing operation, the mold clamping pressure is intensified up to a certain pressure for controlling the mold clamping pressure. Also, in the injection compression molding method, after the injection filling is completed, the mold clamping pressure rises and falls while applying the injection holding pressure. As a result, those methods have been adversely affected by the fluctuation of pressure and the accuracy in the control of a pressure control valve in a hydraulic circuit.
Further, even in the invention disclosed in the foregoing publication, the pressure control is necessarily conducted with accuracy, and there is a case where a pressure balance between the primary and secondary injection pressures both acting as the mold opening force and the mold clamping force is hard to adjust.
Therefore, in such a molding method, the injection pressure is difficult to control, and for the purposes of keeping the index of double refraction within a specification value in the molding of a thin substrate as in a disc having a thickness of 0.6 mm, it is necessary to set the temperature of the die to a value close to a thermal deformation temperature (120.degree. C. or higher in the case of a polycarbonate resin) of resin, and also to have a long cooling period. However, even in this case, there is a limit that the index of double refraction of a completed product is restrained within a range of 60 to 80 nm at the best in a double pass. Moreover, the mechanical characteristics such as the tilt (a microscopic warp of the laser beam incident and reflected portion of the substrate) of the product, etc. remarkably exceed a standard value.
Furthermore, the disc molding technique is under the circumstances where the specification of a high-density disc becomes more strict in the future.